1. Field of the Invention
The present invention relates to an electron multiplying CCD image pickup device used in devices such as cameras and, in particular, to calibration with respect to changes in a characteristic of the electron multiplying CCD image pickup device.
2. Description of the Related Art
Electron multiplying CCD (Charge Coupled Device) image pickup devices that use impact ionization as a method for improving the sensitivity of solid-state image pickup devices have been disclosed (see Japanese Patent Laid-Open No. 7-176721 and No. 10-304256, for example).
FIG. 6 shows an example of such electron multiplying CCD image pickup devices. Referring to FIG. 6, the electron multiplying CCD image pickup device has an image pickup area 903, a storage area 904, a horizontal CCD register 905, an electron multiplying CCD register 906, and an output amplifier 907.
The image pickup area 903 has a photodiode 901 and a vertical CCD register 902 and generates signal charge (electrons). The storage area 904 accumulates signal charge transferred from the image pickup area 903. The horizontal CCD register 905 receives signal charge from the storage area 904 and transfers them horizontally. The electron multiplying CCD register 906 is connected to the horizontal CCD register 905 and multiplies signal charge provided from the horizontal CCD register 905. The output amplifier 907 converts a signal charge multiplied in the electron multiplying CCD register 906 to a voltage.
FIG. 7 shows a cross-section and an electric potential distribution of the electron multiplying CCD register 906 shown in FIG. 6. Referring to FIG. 7, the electron multiplying CCD register 906 has a gate oxide film 908, horizontal transfer gate electrodes 909, 910, and an electron multiplication gate electrode 911. The horizontal transfer gate electrodes 909, 910 and the electron multiplication gate electrode 911 are placed on the top surface of the gate oxide film 908. The potential distribution of the gate oxide film 908 is schematically shown in FIG. 7.
Operation of the electron multiplying CCD image pickup device shown in FIG. 6 will be described below.
First, signal charge generated by photoelectric conversion in the photodiode 901 is read into the vertical CCD register 902. The signal charge read into the vertical CCD register 902 is vertically transferred through the image pickup area 903 and the storage area 904 to reach the horizontal CCD register 905. The signal charge is further horizontally transferred through the horizontal CCD register 905 to be sent to the electron multiplying CCD register 906.
In the electron multiplying CCD register 906, a high voltage is applied to the electron multiplication gate electrode 911 to generate a large potential difference (high electric field) as shown in FIG. 7.
The signal charge 912 which has reached the electron multiplying CCD register 906 is accelerated by the large potential difference and collides with a silicon crystal lattice. The collision causes impact ionization, which generates a new pair of an electron 913 and a hole 914. The hole 914 of the generated pair flows toward a silicon substrate and disappears, whereas the electron 913 is captured in a well.
In practice, several hundred electron multiplication gate electrodes are provided and the process described above is repeated at the several hundred electron multiplication gate electrodes to multiplying the signal charge.
FIG. 8 is a graph of actual measurements of the electron multiplication gain of an electron multiplying CCD image pickup device. Curve 915 shown in FIG. 8 represents the relationship between electron multiplication gate voltage and electron multiplication gain. As can be seen from curve 915, the electron multiplication gain is X1,024 when the electron multiplication gate voltage is 19.2 V.
However, the electron multiplication gate voltage which provides a predetermined electron multiplication gain in the electron multiplying CCD image pickup device as described above can shift toward higher values over time. Curve 916 shown in FIG. 8 shows the voltage of curve 915 that shifted with time. Curve 916 represents actual measurements of the electron multiplication gain after an accelerated test for approximately 100 hours.
Curve 916 is shifted from curve 915 in parallel toward higher voltages in the electron multiplying gate voltage by approximately 2 V. Before the time-course change, a same set voltage of 19.2 V provided a gain of X1,024, whereas, after the time-course changes, a set voltage of 19.2 V provided an electron multiplication gain of zero. The time-course change in the characteristic of electron multiplying CCD image pickup devices has inhibited stable operation of cameras.
Some researchers says the time-course changes in the characteristic of electron multiplying CCD image pickup devices occur because some of signal charges 912 (known as hot electrons) accelerated by the large potential difference or some of electrons 913 or holes 914 generated by impact ionization enter the gate oxide film 908 and are trapped in deep levels. However, the clarification of the real cause and a solution to the problem have not yet been accomplished.